The ABC understands the vote was close, with Senator Hanson-Young winning the preselection vote ahead of Mr Simms on a 53-47 two-candidate preferred basis.

Mr Simms, who was parachuted into the Senate in September 2015 following the resignation of senior Green Penny Wright, lost his position in the 2016 double dissolution election after running second on the ticket behind Senator Hanson-Young.

In a statement, he congratulated Senator Hanson-Young for her preselection.

"This preselection was a lively contest of ideas that engaged a record number of our members and I'm proud to have been able to play my part," Mr Simms said.

"Participatory democracy is one of the four pillars of the Greens and I believe our democratic culture has been strengthened as a result of this process.

"I want to thank the many Greens members who voted for me in this ballot and have supported me throughout my political journey."

Sources have alleged Senator Hanson-Young did not help the party during the state election campaign earlier this year, and was overseas at World Economic Forum events in Davos, Switzerland and Harvard University in the United States.

When questioned whether she ever feared losing the top spot on the party's ticket, she said "no".

"Together, the Greens team and the SA community will fight to protect our environment, to care for people and to support our local artists and creators," Ms Hanson-Young said.

"We will fight to save the Murray, to stop drilling in the Great Australian Bight, to take action on climate change and increase our investment in renewable energy."

In his letter to party members in March, Mr Simms appealed for a refresh of the Senate ticket, saying it was an opportunity to reconnect with South Australians who had switched off from the Greens.

"Our state is ripe for a progressive revolution and the Greens are the party to deliver it. This should be our time," he said.

The Greens have announced Ngarrindjeri elder Major 'Moogy' Sumner will be the party's candidate for Mayo, also being contested by former Centre Alliance MP Rebekha Sharkie and Liberal Georgina Downer.